This invention relates to voltage and current sensing.
Capacitively-coupled voltage measurement is frequently used to measure the voltage present on a high voltage conductor in high voltage alternating current electric systems. Typically, a high voltage capacitor is connected between the high voltage conductor and the secondary winding, and a load capacitor is connected between the secondary winding and the toroidal ferro-magnetic core. The high voltage capacitor and the load capacitor form a simple capacitive voltage divider from which the voltage of the high voltage conductor may be determined. Voltage measurement is often supplemented with a measurement of current flowing through the high voltage conductor. Typically, a current transformer is used to provide this current measurement by surrounding the high voltage conductor with a ferro-magnetic transformer core around which an insulated secondary winding is wound uniformly.
Although capacitively-coupled voltage sensing is widely used, the cost and precision of the capacitively-coupled sensors are closely related to the quality of the high voltage capacitors used to perform the measurements. High precision is often achieved by using closely matched foil capacitors immersed in a dielectric liquid or ceramic capacitors built with high-performance, temperature-compensating materials. These high precision capacitors generally are quite expensive.
A low cost approach is achieved by constructing a voltage-sensing capacitor as an integral part of the high voltage apparatus. The capacitance of such a capacitor is determined by the internal device geometry and the dielectric constant of an associated insulating material. The low cost approach often produces a relatively low capacitance value that limits the overall measurement accuracy of the design. Low capacitance, and therefore low energy, also presents a challenge in transmitting the measured information from the sensor to the device that is performing the voltage measurement.
Parasitic capacitance between the current transformer secondary winding and the high voltage conductor may elevate the potential of the secondary winding, which may lead to failure of the secondary winding insulation. A similar problem applies to the ferro-magnetic based transformer core if the potential is left freely floating with respect to the high voltage conductor potential. To reduce or eliminate this current transformer failure mechanism, the standard approach has been to ground the current transformer core or to add a grounded shielding electrode that protects the current transformer secondary winding.
In one general aspect, simultaneous measurements of voltage and current present on a primary high voltage conductor are achieved through use of a current transformer. Capacitive coupling between the high voltage conductor and the current transformer secondary winding is used to transmit primary voltage information without affecting the current normally flowing through the current transformer secondary winding. The voltage and current information is separated, the voltage information is used to provide a voltage measurement output representative of the voltage present on the primary conductor, and the current information is used to provide a current measurement output representative of the current flowing on the high voltage conductor. The same approach may be used for each phase in a multi-phase system. Thus, capacitively coupled voltage measurement may be combined with current measurement in a single device. The approach exploits the parasitic capacitance normally present between the high voltage conductor and the current transformer secondary winding, and the parasitic capacitance normally present between the current transformer secondary winding and the ferro-magnetic core to form a simplified capacitive voltage divider from which the voltage of the high voltage conductor may be determined.
Implementations may include one or more of the following features. For example, the current measurement may be obtained through an electronic circuit or a secondary transformer, and the voltage measurement may be obtained through an electronic circuit. A neutral current measurement may also be obtained, for example, through an electronic circuit or a transformer.
A capacitive voltage divider may be used in providing the voltage measurement. In multi-phase systems, each phase would have a corresponding capacitive voltage divider. The capacitive voltage divider may have first and second capacitances, where the first capacitance is between the high voltage conductor and the secondary winding of the current transformer and the second capacitance is between the secondary winding of the current transformer and the transformer core. The first and second capacitance may be, for example, the parasitic capacitance between the high voltage conductor and the secondary winding of the current transformer and the parasitic capacitance between the secondary winding of the current transformer and the transformer core of the current transformer connected to a reference potential. A ground potential may be used as a reference potential. In another implementation, any potential may be used as the reference potential. For example, any potential with a voltage difference with respect to the high voltage conductor being measured may be used. In one implementation, the second capacitance may be increased by adding an external capacitor between the current transformer secondary winding and the reference potential. In another implementation, the second capacitance may be increased by adjusting the parasitic capacitance. The parasitic capacitance may be adjusted, for example, by adjusting the device geometry. In one implementation, the second capacitance has a value from approximately 0.001 to 10 microfarads and forms a high pass filter network in combination with a drain resistor that is connected between the current transformer secondary winding and ground, where the cutoff frequency of the high pass filter network may be set between approximately 1 to 0.001 hertz.
An electronic circuit may be used in providing the voltage measurement. For example, the electronic circuit may have an operational amplifier, a resistor connected to the operational amplifier and a terminal of the current transformer, and a drain resistor connected to the operational amplifier.
In another implementation, an auxiliary transformer may be used to provide the current measurement. Alternatively, an electronic circuit may be used to provide the current measurement. The electronic circuit may include an operational amplifier connected to the current transformer and a burden resistor connected to the operational amplifier.
In a further implementation, the neutral current of a multi-phase system may be measured. For example, a transformer with a separate winding for each phase may be used to provide the neutral current measurement. Alternatively, the neutral current may be measured using an electronic circuit.
Another implementation includes canceling from the voltage measurement crosstalk induced by one or more additional phases in a multi-phase system. For example, voltage measurements may be obtained for the additional phases, a product may be generated for each additional phase by multiplying the additional phase voltages by a corresponding predetermined constant, and the product for each additional phase may be subtracted from the voltage measurement. In one example, there are three phases in the multi-phase system.
The crosstalk may be cancelled by an electronic circuit. The electronic circuit may include an operational amplifier, a connecting resistor connected between the input and output of the operational amplifier, and a resistor associated with an additional phase connected to the operational amplifier. In another implementation, the crosstalk may be cancelled by computer software.
The current transformer secondary winding may be protected from insulation failure induced by a transient voltage. For example, a surge suppressor may be connected between the transformer secondary winding and ground.
Although primarily intended for medium voltage power systems, simultaneous measurement techniques may be applied to other voltage levels and system frequencies. Moreover, by reducing the number of components required, the techniques offer a very low cost solution for combined current and voltage measurement. Components for implementing the approach may be retrofitted to existing systems to add voltage sensing capability to older transformer installations. The techniques may be used in a multi-phase system, such as a three-phase system, or in a single phase system.
Other features and advantages will be apparent from the description and drawings, and from the claims.